This invention relates, in general, to friction devices, such as brakes, clutches and friction drives where a high coefficient of friction with high contact load is critical.
This invention is particularly directed to a method and apparatus for transferring torque to drill pipe or casing, such as by manual or power tongs or other gripping assemblies, used on the rig floor of an oil and gas drilling or production rig either onshore or offshore.
More specifically, this invention is an improvement in the grippers on the jaws of such torque transfer apparatus and an improvement in the method of transferring torque to drill pipe or casing.
In known friction devices, the coefficient of sliding friction K is determined by the pairs of materials used and it is common to use hardened sharp steel teeth on the faces of the grippers against the softer pipe material so that the teeth are forcibly clamped around the pipe. The greater the clamping force, the better the grip on the pipe to avoid slippage, By penetrating and damaging pipe surfaces, these teeth developed a K equal to 0.5 to 1 or greater. Unfortunately, metal is chiseled out of the pipe if slippage occurs. The penetration of the pipe by the teeth and the slippage, if the latter occurs, sacrifice tool joints and the body of the pipe.
Another tendency in the prior art is to make the grippers out of a material that will provide minimum wear to the grippers if slippage occurs between the grippers and the pipe. Knowing the limitations of the coefficient of friction, prior art developed in the direction of increased contact area and evenly distributed normal loads, such as in the U.S. Pat. Nos. 4,836,064 and 4,869,137 by D. T. Slator. In the devices described by these patents, rubber backing is used to more evenly distribute normal loads on the pipe. In this way, with relatively low coefficient of friction, greater normal load can be applied with a reduced negative effect on the pipe.
Other gripping arrangements used to achieve the same effect comprises steel needles flexibly bonded together by rubber so that, under normal load, the shifting of the needles redistributes contact pressure.
It is also recognized that in the prior art, a pair of different materials with high coefficient friction is used, for example, composite brake pads against a steel disk. Both the pads and the disks wear out although the pads wear the most and are considered replaceable.
It is also known from friction physics that a majority of materials have a higher dry coefficient of friction against themselves than against other materials. Materials of the same origin have a tendency under load to form a bond similar to a cold weld and when there is slippage between the materials, galling occurs where small particles of the material are removed from the surfaces and jammed between the moving parts and in the prior art, there have been a full range of measures used to prevent galling from happening.
This invention, on the other hand, can be said to be an alternative to the prior art of friction gripping, ie, hard gripping surfaces against soft pipe surfaces and goes in the opposite direction to the prior art developments by providing a material softer than the pipe which contacts the pipe and wherein bonding becomes a desirable feature. In this invention, the transfer of rotational torque to the pipe then becomes a function of the shear strength of the softer material and, if slippage occurs, the softer material is damaged rather than the pipe.